1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as copiers, facsimile machines and printers, and more particularly to an electrophotographic image forming apparatus including an image carrier capable of bearing an electrostatic latent image and a developer carrier capable of bearing a developer and supplying the developer to the image carrier to develop the electrostatic latent image.
2. Discussion of the Background
Currently, various developing methods are known for electrophotographic image forming apparatus. Among the developing methods, contact-type developing methods, which are disclosed, for example, in Japanese Laid-Open Patent Publication No. 9-73229, are well known and widely used because they can produce images having good dot-image reproducibility. In a typical contact-type developing method, a developing roller, which serves as a developer carrier and which bears a developer thereon, develops an electrostatic latent image formed on an image carrier while contacting the image carrier to visualize the electrostatic latent image.
However, since the image carrier contacts the developing roller on which a toner layer is held, a problem which tends to occur is that the toner held on the developing roller adheres to non-image areas of the image carrier by van der Waals forces between them, and/or a reversely charged toner included in the developer adheres to non-image areas of the image carrier by an electrostatic force, resulting in formation of background fouling in the resultant toner images formed on the image carrier.
In attempting to solve this background fouling problem, various methods have been disclosed. For example, a method is disclosed in which a pressure of contact of a developing roller with an image carrier is increased. In the contact-developing methods, the rotation of an image carrier tends to be affected by vibration of a developing roller which is caused by the vibration of a driving device and/or a drive-transmitting device which drive the developing roller, and thereby the image carrier rotates unevenly, resulting in formation of so-called "banded fouling", which is fouling like horizontal stripes, in the resultant toner image. When the pressure of contact of the developing roller with the image carrier is increased under such circumstances, a problem which occurs is that serious banded fouling is observed in the resultant toner images.
In attempting to solve the background fouling problem, a method is disclosed in which a linear velocity of rotation of a developing roller is set to be relatively high compared to that of an image carrier. For example, Japanese Laid-Open Patent Publication No. 9-73229 discloses a method in which a developing roller having a one-component developer layer thereon contacts an image carrier to develop an electrostatic latent image formed on the image carrier, wherein the rotation of the developing roller is set so as to be from 1.2 to 3.0 times, preferably from 1.5 to 2.5 times, as high as that of the image carrier. However, the increase of rotation of the developing roller causes not only banded fouling, but also another undesired image, so-called "toner deviation in solid toner images" in which a rear end of a solid image has a relatively high image density compared to the other portion of the solid image.
In addition, in attempting to solve the background fouling problem, Japanese Laid-Open Patent Publication No. 8-254933 discloses an image forming apparatus having a toner density detecting device which detects a toner density of non-image areas of an image carrier and a lubricant-coating controlling device which controls an amount of a lubricant to be coated on the image carrier based on the information of the toner density detected by the toner density detecting device. The coating amount of the lubricant is controlled by adjusting the pressure of contact of the lubricant coating device with the image carrier. However, when the contact pressure of the lubricant coating device is increased, banded fouling problem, which occurs by the same mechanism as in the case mentioned above, tends to occur.
Because of these reasons, a need exists for an image forming apparatus that can produce images having good image qualities without background fouling such as banded fouling.